Oxide ion conductors are a material which attracts attention as a functional ceramic usable in various electrochemical devices such as solid electrolytes of batteries such as fuel cells (SOFC), ion batteries, and air cells, sensors, and separation membranes.
Hitherto, as oxide ion conductors, Perovskite type oxides such as LaGaO3 and the like have been widely known as well as ZrO2 having a fluorite type structure, in particular, stabilized ZrO2 doped with Y2O3 has been widely used.
A number of the oxide ion conductors of this type which have hitherto been known are a defect structure type in which an oxygen defect is introduced and an oxygen ion moves through this oxygen defect. In contrast, apatite-type oxide ion conductors such as La10Si6O27 have been recently reported as an oxide ion conductor in which interstitial oxygen moves.
With regard to the apatite-type oxide ion conductor, for example, Patent Document 1 (JP 2004-244282 A) discloses an oxide ion conductor which contains a trivalent element A, a tetravalent element B, and oxygen O as constituent elements, has a composition formula represented by AXB6O1.5X+12 (where 8≤X≤10), is composed of a composite oxide having an apatite-type crystal structure, and has an anisotropic oxygen ion conductivity.
Among such apatite-type oxide ion conductors, a lanthanum silicate-based oxide ion conductor is known as a solid electrolyte which exhibits high ion conductivity in the intermediate temperature region, and for example, a composition formula of La9.33+xSi6O26+1.5x or the like attracts attention.
A lanthanum silicate-based oxide ion conductor has an apatite structure exhibiting low symmetry, namely, high anisotropy and low activation energy for ion conduction, and it is thus said to be advantageous particularly for low temperature operation in the case of being used as a solid electrolyte of SOFC.
With regard to the lanthanum silicate-based oxide ion conductor of this type, for example, Patent Document 2 (JP 8-208333 A) discloses an oxide ion conductor which contains LnXSi6O(3X/2)+12 (where Ln is a trivalent rare earth element of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, or Dy, and x is 6<x<12) as the main component and has a crystal system of the main constituent phase of the sintered body calcined at 1300° C. or higher consisting of a hexagonal crystal.
In addition, Patent Document 3 (JP 11-71169 A) discloses an oxide ion conductive ceramic which is a sintered body containing (RE2O3)x(SiO2)6 (RE is an element selected from La, Ce, Pr, Nd, and Sm, and x satisfies the condition of 3.5<x<6) calcined at a temperature of 1700° C. or higher as the main component and has an apatite crystal structure as the main constituent phase.
However, it is possible to expect to further enhance the ion conductivity through orientation since the lanthanum silicate-based oxide ion conductor exhibits anisotropic ion conductivity.
As a manufacturing method capable of orienting a lanthanum silicate-based oxide ion conductor in one direction, a method to fabricate a single crystal of LSO by a floating zone method (FZ method) or the like or a method which a La2O3 powder and a SiO2 powder are mixed together and then subjected to a heat treatment at from 700 to 1200° C. to produce a porous material of a composite oxide, this porous material is pulverized into a powder, the powder is then mixed with a dispersion medium to form a slurry, this slurry is solidified in the presence of a magnetic field to form a molded body, this is then sintered at from 1400 to 1800° C., thereby obtaining an ion conductive oriented ceramic in which the orientation directions of crystals are roughly matched has been proposed.
In addition, Patent Document 4 (JP 2011-37662 A) discloses a method for manufacturing an ion conductive oriented ceramic in which first, an oxide raw material containing an oxide powder of a lanthanoid and an oxide powder of at least either of Si or Ge is mixed (oxide raw material mixing step S1), the mixed oxide raw material is then heated and melted to be in a liquid state, this is casted and then rapidly cooled to obtain a glassy material G (melting vitrification step S2), and subsequently the glassy material G is crystallized through a heat treatment at from 800 to 1400° C. (crystallization step S3) in order to provide a method for manufacturing an ion conductive oriented ceramic by which a large one can be easily obtained and ion conductivity can be improved although it requires a low cost and is a simple process.
Furthermore, Patent Document 5 (JP 2013-184862 A) discloses a method to obtain an apatite-type lanthanum silicogermanate polycrystalline substance by heating a bonded body obtained by bonding a first layer containing La2Si2O7 as the main component, a second layer containing La2[Si1-xGex]O5 (where x represents a number in the range of from 0.01 to 0.333) as the main component, and a third layer containing La2S2O7 as the main component in the order of the first layer/the second layer/the third layer at a temperature at which element diffusion occurs and removing the layers other than the layer that is positioned at the most intermediate position in the laminated structure generated after heating.
Patent Document 1: JP 2004-244282 A
Patent Document 2: JP 8-208333 A
Patent Document 3: JP 11-71169 A
Patent Document 4: JP 2011-37662 A
Patent Document 5: JP 2013-184862 A